1. Field of the Invention
The present invention relates generally to a fluorescent lamp and more particularly to a fluorescent lamp having an improved composite phosphor layer.
2. Description of Related Art
There are two principal types of phosphors used in fluorescent lamps: relatively inexpensive halophosphors, and relatively expensive rare earth phosphors. Halophosphors, though commonly used due to their low cost, exhibit poor color rendering properties and lower lumens compared with more expensive rare earth phosphors. Rare earth phosphors, for example blended into a triphosphor layer as is known in the art, exhibit excellent color rendering properties and high lumens but are used sparingly due to their high cost.
The fluorescent lighting industry has adopted a dual-coating technology for producing certain medium performance lamps incorporating both halophosphors and rare earth triphosphors. xe2x80x9cMedium performancexe2x80x9d as used herein means performance (in terms of color rendering properties and lumens) intermediate between that of inexpensive halophosphors and expensive rare earth triphosphors. The dual-coating technology involves applying halophosphors and rare earth triphosphors as discrete coating layers with the more expensive triphosphor layer placed in the well-utilized second coat next to the arc discharge. Medium performance fluorescent lamps produced using this dual-coating technique have become quite popular and account for between 70%-90% of fluorescent lamp sales worldwide.
Despite the popularity of this dual-coating technology, the application of phosphors as discrete layers presents many significant manufacturing problems. Initially, the expensive triphosphor layer is very thin, often less than a monolayer of particles, contributing to significant variations in thickness and uniformity of the triphosphor layer during the application process. Such variations result in increased variations in the color rendering index (CRI) and lamp brightness which are strongly related to the triphosphor layer thickness.
Other manufacturing difficulties include a narrow range of acceptable coating additives (such as dispersants and surfactants), as well as elevated coating and production costs. Each coating step increases production losses and requires significant equipment and labor usage.
In addition to two discrete phosphor layers, fluorescent lamps of the prior art require a third discrete boundary layer of alumina particles coated directly onto the glass tube beneath the phosphor layers. This third layer of alumina prevents UV emission from the fluorescent lamp by reflecting unconverted UV radiation back toward the interior of the lamp where it is subsequently converted to visible light by the phosphors. The alumina layer also minimizes mercury loss due to reaction with the glass tube. The addition of this third coating layer further increases production losses due to equipment and labor usage.
There is a need in the art for a lamp that combines halophosphors, rare earth phosphors or triphosphors and alumina particles into a single blended composite coating that can be applied as a single layer in a single step in the production of medium performance fluorescent lamps.
A mercury vapor discharge lamp is provided comprising a light-transmissive envelope having an inner surface, means for providing a discharge, a discharge sustaining fill of mercury and an inert gas sealed inside the envelope, and a single composite layer coated on the inner surface of the envelope. The composite layer is provided is having at least one type of halophosphor, at least three types of rare earth phosphors, and colloidal alumina particles in a heterogeneous mixture.